Hello everyone :)

Currently I am running campaign related to 7 Deadly Sins. Soon PCs will start dealing with first of them - Gluttony. I've decided that this demon will has lair in Baldur's Gate's sewers. And he is living there for 2 years. What do you think - how his presence influence on city and on inhabitants? I am looking for some adventures hooks, I don't want to send them to city and kill that guy in 1st session connected to Gluttony. Thanks for any help :)

Oozes are a good way to go! They make sense for a sewer environment and it just needs a stronger version of one to be your evil guy with ooze minions. The ooze could occasionally come over to the surface and make these giant holes that it comes out of, as the ooze kidnaps and devours people. People will be scared of living close to big sewer networks maybe as a response.

Also there is a Volos guide monster called a devourer who is a real glutton for souls! Although it is CR 13 (could always nerf the stats by a lot though to get good CR, or run a scenario where they can beat the devourer not with a full blown combat which would most likely be TPK but by using it’s flaws against it) You could also have the devourer spawn a different creature that isn’t an undead if you don’t want that aspect. Maybe have the devourer be a form of urban legend about a horrid creature that attacks gluttonous creatures to fuel it’s own gluttony during dark nights, until an attack which the players witness where the creature instead of hiding amongst shadows, now arises. There could also be superstitious communities where they fear the devourer/heed it.

Troglodytes are low CR and I recall them worshiping a demon lord-toad dude, who could have a weaker manifestation or servant as the demon! Their impact could be occasional surface raids, which results in the city hiring adventurers to deal with them and drive out the monsters.

There is a annual Patriar dinner party hosted by the Gate’s top foodie, where all the nobles dine on exquisite dishes from all around Faerûn (Amnish Molasses Blackbread, Luiren Spring Cheeze, Knucklehead trout, Calimshan Curry, Cormyrean Roast Lamb, Underdark Rothe Steak, etc) and dress up like it’s The Carnival of Venice. And somebody plans to steal the hosts secret family recipe passed down from his great great grandmama. Only tonight is different, the demon is influencing the party tonight. The demon probably looks like ‘Meatsweats’ from the new ninja turtles cartoon. Both the guests and thieves begin eating and drinking like it’s now their addiction. They can’t stop. As the food slowly runs out the party goers begin feasting on anything they can eat, including each other (use zombie stats for the gluttony warped party goers) so the PC’s better hurry. Fortunately the host knows that grandmas original dessert recipe when made properly can dispel the demons magical hold over the guests and restore them to their annoyingly Posh selves. Only it’s in a secure hidden room in the basement of his mansion .... which is down in the sewers. 

- so party it up as either “invited guests“ or “possibly hired thieves”, find the recipe once things turn south, help the host/chef cook the recipe with skill checks, save the party goers and get a glimpse of the demon causing all the gluttony while they are down in the sewers as it flees to its hideout. 

Maybe a wealthy merchant or minor lord has been developing a taste for human flesh? Fresh bodies have been disappearing from local churches, morgues, and funeral parlors, and a local investigator or reporter wants to get to the bottom of it. They need some tough folks to follow the trail and make sure justice is done, and the adventurers look like the ones to do it. But it won’t be easy, as the noble employs a number of hired thugs and guard monsters. When they’ve defeated the evil noble, his butler informs them that he started acting very strange about two years ago, making frequent trips into the sewers under his house.

If the players don’t investigate the sewers yet, they might hear rumors of a tavern where a half dozen people have drank themselves to death within a month. Showing up to sample the beer, they find several bloated bodies dumped in the alley outside. Turns out the bartender’s been enchanting the beer so people drink themselves to death, and the only antidote is in a dangerous hill pass just outside town. There might be a dungeon or cave in which the rare plant is hidden. After recovering the antidote and defeating the bartender, the characters learn from his cook that he started acting strangely and disappearing into the sewers on and off about two years ago.

By now, the characters should realize that something is up in the sewers, and you can let them take the initiative and explore them themselves. It’ll pretty quickly become clear that there’s an evil presence, as cultists and fiends accompany the regular rats and oozes, all of whom are fattened and worshippers of gluttony. There they can finally confront the gluttony avatar, and they should be high-level enough to do so!

The citizenry hasn't been storing any food for the upcoming winter because they've been overeating.

Wine prices have doubled from the usual PHB cost due to demand.

Some people are rail thin (those dependent on charity), but many more are overweight.

Vegetable prices are down at the market because the demand is for sweets.

The leadership is morbidly obese.

The prostitutes take exotic or luxurious foods in place of cash.

Taverns hire private security, not for the dining area, but for the pantry and kitchen areas.

You could add a nupperibo. That's if your party is low level.

Have everyone be hungry all the time. Make the character make regular saving throws or they have to drop whatever they are doing and eat. 

Read Dante's Inferno and play the video game if you can. it will give you some ideas. You could just watch a play-through on youtube or something as well